The present application claims priority to Japanese Application No. P10 187774 filed Jul. 2, 1998, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a method and apparatus for measuring the structure of an object by heterodyne detection or homodyne detection employing the laser light.
2. Description of the Related Art
For measuring the structure of an object, an optical microscope has so far been used extensively. In an ordinary optical microscope, the observing light is illuminated on the object for measurement, and observation is made of the intensity distribution of the observing light transmitted through the object or that of the light reflected from the object for measurement. The limit of resolution of the optical microscope is determined by the limit of optical diffraction. That is, if the wavelength of the light for observation is xcex and the numerical aperture of the object lens is NA, the spatial wavelength of the limit of resolution in an optical microscope adapted for observing only the light intensity distribution is expressed by xcex/2xc3x97NA. In such optical microscope, the light for observation is extremely weak in the vicinity of the limit of resolution to render the observation difficult.
On the other hand, there is proposed a technique of measuring the structure of the object by heterodyne or homodyne detection as a technique which enables the measurement of a fine textured structure even with the weak light. With the heterodyne or homodyne detection, it becomes possible to measure the fine textured structure with the weak light by employing the laser light superior in coherence as the light for observation and by exploiting the phase information of the laser light.
The principle of the technique of exploiting the phase information of light is disclosed in, for example, The Antenna Properties of Optical Heterodyne Receivers, Appl. Oct., vol.5 (1966) 1588 to 1594. There is also disclosed in Probing of Acoustic Perturbations by Coherent Light Appl. Output terminal, vol.8 (1969) 1572 to 1573 a technique of measuring the intensity and the phase of the reflected light from the object under measurement by exploiting wavelength shift by an acousto-optical element of a carrier signal f1 and by synchronously detecting beat signals of the frequency f1.
There is disclosed in U.S. Pat. No. 3,796,495 entitled:xe2x80x9cApparatus and Method for Scanning Phase Profilometryxe2x80x9d a technique of differential heterodyne detection of two transversely shifted beams. There is disclosed in U.S. Pat. No. 4,171,159 entitled: xe2x80x9cOptical Homodyne Microscopexe2x80x9d a technique of effecting homodyne detection with mechanical phase modulation by a piezoelectric element. There is disclosed in U.S. Pat. No. 4,353,650 entitled: xe2x80x9cLaser Heterodyne Surface Profilerxe2x80x9d a technique of illuminating both the reference light and the detection light on the object for measurement and causing rotation of the object about the reference light as center of rotation to effect phase measurement. In the U.S. Pat. Nos. 4,627,730 and 4,848,908, there is disclosed a technique of illuminating both the reference light and the detection light on the object for measurement and employing a common optical path to improve resistance against oscillations. There is disclosed in Japanese Laying-Open Patent H-7-248203 a laser scanning microscope exploiting the heterodyne detection.
There is further disclosed the result of observation of a pseudo-living body sample by heterodyne detection in the visible light range in a thesis entitled: xe2x80x9cMeasurement of Spectroscopic Transmission Characteristics in the Visible Range to the Near-Infrared Range of a Pseudo-Living Body Sample Employing Optical Heterodyne Detection Methodxe2x80x9d in Optics Vol.27.1 (1998) 40 to 47.
By exploiting heterodyne or homodyne detection, it becomes possible to measure the fine textured structure with weak light. Of course, it would be meritorious if measurement can be made of a micro-sized structure. It would be more meritorious if measurement can be made not only of the planar direction but also of the depth-wise direction of the object for measurement.
It is therefore an object of the present invention to provide a method and apparatus for measuring the structure of an object for measurement by exploiting heterodyne or homodyne detection, whereby measurement can be made of a micro-sized structure of the object for measurement.
It is another object of the present invention to provide a method and apparatus for measuring the structure of an object for measurement by exploiting heterodyne or homodyne detection, whereby measurement can be made of a structure along the depth-wise direction of the object.
In one aspect, the present invention provides a measurement device including ultraviolet laser light generating means for generating ultraviolet laser light by wavelength conversion of laser light from a solid-state laser light source, and measurement means for measuring the structure of an object for measurement by heterodyne detection or homodyne detection employing the ultraviolet laser light. Specifically, the ultraviolet laser light means the laser light having the wavelength of the order of 180 to 360 nm.
Preferably, the solid-state laser light source is oscillated in a single longitudinal mode.
Preferably, the solid-state laser light source is a diode laser pumped solid state laser pumped by the laser light from a semiconductor laser to radiate laser light. The diode laser pumped solid state laser preferably includes a monolithic ring type optical resonator. The laser light from a semiconductor laser falls on the monolithic ring type optical resonator to excite the laser medium to radiate the laser light. The diode laser pumped solid state laser is preferably configured so that the optical path in the monolithic ring type optical resonator is non-coplanar.
Preferably, the solid-state laser light source includes semiconductor laser and a wavelength selecting element and the laser light from the semiconductor laser is radiated via the wavelength selecting element to radiate the laser light of a single frequency.
The ultraviolet laser light generating means preferably generates the ultraviolet laser light by wavelength conversion by multiple stages.
As the wavelength converting means, a nonlinear optical element, formed as a ring type resonator, is desirable. The laser light from the solid-state laser light source is resonant in the nonlinear optical element, and the nonlinear optical element generates harmonics or the sum frequency to effect the wavelength conversion.
The wavelength converting means preferably includes an optical resonator made up of multiple mirrors and a nonlinear optical element arranged in the optical resonator. The laser light from the solid-state laser light source is resonant in the optical resonator, with the nonlinear optical element generating harmonics or the sum frequency to effect the wavelength conversion. Preferably, the position control means precisely controls the position of the mirrors making up the optical resonator.
Preferably, the measurement means includes movement means for causing movement of an object for measurement. The movement means causes movement of the object for measurement so that a light spot of the ultraviolet laser light scans the object for measurement at the time of measuring the structure of the object for measurement.
Preferably, the measurement means includes deflection means for deflecting the ultraviolet laser light to control the proceeding direction of the ultraviolet laser light. The deflection means causes deflection of the ultraviolet laser light so that the ultraviolet laser light will be incident on a pre-set position of the object for measurement at the time of measuring the structure of the object for measurement. Alternatively, the deflection means causes deflection of the ultraviolet laser light so that a light spot of the ultraviolet laser light will scan the object for measurement at the time of measurement of the object for measurement.
Preferably, the measurement means includes light splitting means for splitting the ultraviolet laser light into the detection light illuminated on the object for measurement and reference light for heterodyne or homodyne detection.
For heterodyne detection, the measurement means preferably includes frequency shifting means for frequency shifting at least one of the detection light and the reference light. The frequency shifting means preferably is an acousto-optical modulator.
For homodyne detection, the measurement means preferably includes phase shifting means for shifting the phase of at least one of the detection light or the reference light. The phase shifting means is preferably an electro-optical phase modulator. The phase shifting means preferably includes a mirror arranged on an optical path of the detection light and/or the reference light and a mirror position controlling means for controlling the position of the mirror. The mirror position controlling means controls the position of the mirror by mirror phase control means for shifting the phase of the detection light and/or the reference light.
In splitting the ultraviolet laser light by light splitting means into the detection light and the reference light, the light splitting means preferably splits the ultraviolet laser light so that the power of the reference light will be larger than that of the detection light. Specifically, the light splitting means splits the ultraviolet laser light so that the power of the reference light will be not less than 100 for the power 1 of the detection light. If the object for measurement is susceptible to damages by the ultraviolet light, the light splitting means splits the ultraviolet laser light so that the power of the detection light illuminated on the object for measurement will be not larger than 1 xcexcw, with the reference light power being larger than the detection light power.
Preferably, the measurement means includes a photodetector for receiving the return light reflected form the object for measurement and the reference light used for heterodyne or homodyne detection, the photodetector detecting a heterodyne signal or a homodyne signal produced on interference between the return light and the reference light. The photodetector is preferably an Si-PIN photodiode, an Si-APD photodiode or a GaN photodiode.
The measurement means preferably forms multiple detection light spots on the object for measurement, with the return light of the spots being detected by multiple photodetectors. These photodetectors are preferably Si-PIN photodiodes, Si-APD photodiodes or GaN photodiodes.
It is possible for the measurement means to includes an object lens of silica, quartz or fluorite as light converging means for converging the ultraviolet laser light on the object for measurement. The measurement means preferably includes an object lens as light converging means for converging the ultraviolet laser light on the object for measurement and a protective cover for protecting the surface of the object lens.
It is possible for the measurement means to include two ultraviolet laser light generating means for generating two ultraviolet laser light beams having different wavelengths, as the ultraviolet laser light generating means. The measurement means illuminates the first ultraviolet laser light radiated from one of the ultraviolet laser light generating means. The measurement means causes the second ultraviolet laser light radiated from the other ultraviolet laser light generating means to interfere with the return light of the first ultraviolet laser light reflected back from the object for measurement by way of performing heterodyne detection.
In another aspect, the present invention provides a measurement method including generating ultraviolet laser light by wavelength conversion of the laser light from a solid-state laser light source and measuring the structure of an object for measurement by heterodyne detection or homodyne detection employing the ultraviolet laser light.
In the measurement method, preferably the ultraviolet laser light is converged by an object lens on an object for measurement at the time of heterodyne detection or homodyne detection, and liquid is arranged between the object lens and the object for measurement. The liquid may be such one as to undergo chemical reaction with the object for measurement. If the liquid is arranged between the object lens and the object for measurement, an exchangeable protective cover is fitted on the object lens.
In still another aspect, the present invention provides a measurement device including laser light generating means for generating laser light, light splitting means for splitting the laser light from the laser light generating means into multiple light beams, frequency shifting means for applying frequency shifting to the laser light so that the laser light beams split by the light splitting means will be of different frequencies, imaging means for conducting the laser light beams frequency shifted by the frequency shifting means to an object for measurement and for imaging the respective laser light beams at different focal point positions, and measurement means for performing heterodyne detection using the respective laser light beams imaged at different focal point positions by the imaging means. The measurement means separates the heterodyne signals resulting from heterodyne detection into respective frequency bands for measuring the structure of the object for measurement in association with respective imaging points.
In this measurement device, the light splitting means preferably splits the laser light from the laser light generating means into multiple light beams having different optical axes, and the frequency shifting means applies frequency shifting to each of the light beams split by the light splitting means. The light splitting means preferably is an optical block the opposite surfaces of which are mirror surfaces. The laser light incident on the light splitting means is repeatedly reflected between the opposite mirror surfaces of the optical block. The laser light is split each time it is reflected by one of the mirror surfaces into the light transmitted through the mirror surface and the light reflected by the mirror surface. The laser light repeatedly reflected between the two mirror surfaces of the optical block may be the divergent light. In this case, the split light beams have different virtual beam light emitting positions.
The light splitting means may be a first mirror and a second mirror arranged facing the first mirror and adapted for transmitting a portion of the incident light therethrough. The frequency shifting means preferably is a frequency shifter arranged between the first and second mirrors. The laser light incident on the light splitting means is repeatedly reflected between the first and second mirrors. The laser light is split each time it is reflected by one of the mirrors into the light transmitted through the mirror and the light reflected by the mirror. The laser light is frequency shifted by a frequency shifter each time it is reflected between the first and second mirrors. The laser light repeatedly reflected between the first and second mirrors may be the divergent light. In this case, the split light beams have different virtual beam light emitting positions.
In yet another aspect, the present invention provides a measurement method including splitting the laser light into multiple laser light beams and frequency shifting the laser light so that the multiple split laser light beams will be of different frequencies, conducting the frequency shifted laser light beams to an object for measurement, imaging the respective laser light beams at different focal point positions, performing heterodyne detection using the laser light beams imaged at the different focal point positions, separating the heterodyne signals obtained by the heterodyne detection into respective frequency bands and measuring the structure of the object for measurement associated with the respective imaging points.
In the measurement method, preferably the ultraviolet laser light is converged by an object lens on an object for measurement at the time of heterodyne detection or homodyne detection, and liquid is arranged between the object lens and the object for measurement. The liquid may be such one as to undergo chemical reaction with the object for measurement. If the liquid is arranged between the object lens and the object for measurement, an exchangeable protective cover is fitted on the object lens.
According to the present invention, as described above, it becomes possible to measure the structure of a finer texture in measuring the structure of an object for measurement using heterodyne detection or homodyne detection. Also, according to the present invention, it is possible to measure the structure along the depth of the object for measurement in measuring the structure of an object for measurement using heterodyne detection.